Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.2.7

3.1.2.7

atha darbhataruṇakamantardadhāti | oṣadhe trāyasveti vajro vai kṣurastatho hainameṣa vajraḥ kṣuro na hinastyatha kṣureṇābhinidadhāti svadhite mainaṃ hiṃsīriti vajro vai kṣurastatho hainameṣa vajraḥ kṣuro na hinasti

Thereupon he lays a stalk of sacrificial grass on (the hair of the whisker), with the text, 'O plant, protect me!' For the razor is a thunderbolt, and thus that thunderbolt, the razor, does not injure him. Thereto he applies the razor, with the text, 'O knife, injure him not!' for the razor is a thunderbolt, and thus that thunderbolt, the razor, does not injure him.